Such a fastening element is already known (German utility model 7 503 898). It is employed for the fastening of ornamental ledges and comprises a head member designed in the form of a ledge retaining means with a sealing screen, said sealing screen lying in close contact against the carrier plate from one side thereof.
A hollow shank portion closed at the bottom and symmetrical with respect to rotation is provided with a restriction so that the edge of the carrier plate opening is received in the restriction.
Such fastening elements which are employed for a concealed, i.e. an invisible, type of fastening and which normally are pre-assembled with the article to be fastened are required to be capable of being simply fixed and easily mounted, they should have high forces of retention and seal the openings in the carrier sheet metal member against moisture. These requirements the above described fastening element meets only partially, especially with a view to the forces of impression and withdrawal, respectively. In the case of the known fastening element these may be varied only by changing the angles of introduction and the angles of undercutting with the ratio of the force of impression to the force of withdrawal being almost equal.
Apart from the known one-piece fastening element also such fastening elements have become known which consist of two cooperating members the one of which being adapted to be fitted in the opening of the carrier plate as an insert sleeve and the other member being designed as a retaining bolt with a head plate for the reception of a hollow ledge, with the retaining bolt penetrating through the sleeve and tightening in the axial direction of movement. In this arrangement it is disadvantageous that in case of repair the insert sleeve must be separately removed from the carrier plate.